This invention in general relates to interferometric apparatus and methods for measuring the wavefront and intensity profile of collimated beams and in particular to the measurement of fiber collimators used in the telecom industry.
Many telecom components, such as switchers, add-drop multiplexers and the like, require access to a free space beam to perform their designated function. Therefore, fiber-to-free-space (collimation) devices and free-space-to-fiber (focusing) devices are required to provide access to the light carrying the information and to couple or recouple light back into the fiber. It is important for telecom component functional efficiency that the free space beam be well collimated and aberration free. An aberrated beam directly affects light to fiber coupling efficiency (commonly known as insertion loss (IL)), leading to unacceptable light loss in the component. To measure IL well, and to characterize currently manufactured collimators, a device to measure the wavefront and intensity distribution of the collimated beam to high precision is needed.
Prior art approaches to collimated beam wavefront measurement include both interferometric and non-interferometric methods. Older non-interferometric methods include the knife-edge and Hartmann tests, but these provide little information on the spatial characteristics of the wavefront or intensity distributions of the collimated beam. Shack-Hartmann sensors are probably the most common non-interferometric approach in use today, providing a modest amount of spatial information, but it is very difficult for Shack-Hartmann sensors to achieve the accuracy and resolution, in both the lateral and vertical dimensions, of interferometric approaches. On the other hand, Shack-Hartmann sensors are inexpensive and do not require coherent sources.
Prior art interferometric approaches include wavefront shearing methods, including both radial shearing and lateral shearing designs. Lateral shearing consists of splitting the test wavefront into two replicas, translating one replica laterally with respect to the first replica and interfering the two replicas. The resulting interferogram can be analyzed to measure the original wavefront. A complete wavefront measurement requires analyzing both vertical and horizontal shears. The method has the disadvantage of sensitivity based on the shear magnitude, two independent measurements that must be combined, and information loss at the wavefront boundary.
Radial shearing consists of splitting the test wavefront into two replicas, magnifying one replica and interfering the two replicas. The advantage over lateral shearing is that image magnification is easy to do optically and only one shear measurement is required. The disadvantages include sensitivity based on the magnification factor and poor sensitivity near the magnification origin. Often, the image magnification factor used is high enough to produce an effectively plane reference wavefront. Both of these approaches can operate with incoherent beams by utilizing an equal path Mach-Zehnder design.
The use of a small area (point-like) apertures or disks to produce reference wavefronts from a diffraction limited point source is known. Here, the reference diffraction spot is produced from the test wavefront by focusing the test wavefront onto a pinhole.
R. G. Klaver, et al. in a paper entitled xe2x80x9cInterferometer to measure the form figure of aspherical mirrors used in EUV lithography,xe2x80x9d Proc. SPIE, Laser Metrology and Inspection, Vol. 3823 (1999) describe an instrument using no bulk optical components to recombine the beams However, this instrument cannot measure collimated wavefronts, and has poor lateral resolution. It requires the use of two fiber tips to act as light beams with known wavefront characteristics. One beam interacts with the optic under test and its wavefront is modified by said optic. The other beam interferes with this modified wavefront, and the interference is analyzed to extract the characteristics of the optic under test.
Accordingly, there continues to be a need for an instrument of simple and inexpensive architecture for analyzing the wavefront and intensity profile of collimated beams, and it is a primary object of this invention to satisfy this need.
Another object of this invention is to provide apparatus and methods by which the wavefront and intensity profile of collimated beams may be measured with high lateral resolution.
Yet another object of this invention is to provide easily aligned apparatus for analyzing the wavefront and intensity profile of collimated beams.
Still another object of the invention is to provide collimated beam wavefront and intensity analyzers that are highly accurate and repeatable.
Yet another object of the invention is to provide collimated beam wavefront and intensity analyzers having minimal optical components.
Another object of the invention is to provide collimated beam wavefront and intensity analyzers having perfectly known reference wavefronts.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter when the description to follow is read in conjunction with the drawings.
The invention consists of an interferometric apparatus and method for measuring collimated test beams. A diverging reference beam is provided and a single bulk optic for combining the collimated and reference beams, preferably in the form of a plate beamsplitter. The reference beam is produced by light emitted from the tip of a single mode optical fiber fed by the same source as is used to generate the test beam. The beamsplitter combines the light from the test and reference beams to produce an interference pattern. The interference pattern is directed through an optical assembly to magnify the region of interference to increase lateral resolution and thereafter onto an detector, preferably of two dimensions, or areal. Means are provided for translating the end of the single mode fiber from which the reference beam emanates or the device under test along the optical axis to enable phase shifting interferometric analysis. Alternatively, phase shifting via wavelength-tuning may be employed.
Analyzing the areal detector data acquired while phase shifting determines the wavefront of the collimated test beam since the reference wavefront shape is known. Apparatus to control the intensity and/or polarization of the beams is provided to facilitate the determination of the intensity profile of the test beam.
In an alternate aspect of the invention, the interference pattern is formed in the plane of a diffusing surface carried on a rotating diffuser disk and is thereafter imaged by a zoom camera for purposes of adjusting magnification to control lateral resolution.